Microwave is a sort of electromagnetic wave which frequency is 300MHz to 300GHz .The water molecule in medium materials is polar molecule,under the effect of the rapid change of electromagnetic field,its polarity orientation will alter with the changes of the external electric field with the high-speed,causing the movement and friction of molecule.so that the microwave field energy can be converted to heat energy in medium to rise the temperature of materials.accomplishing heating ,puffing and a series of physical and chemical processes ,and so ,microwave heating is achieved.